1. Field of the Invention
The present invention relates to a DC offset cancellation circuit that cancels a DC offset voltage occurring between a pair of complementary differential output signals-outputted from a differential amplification circuit, a differential amplification circuit with a DC offset cancellation circuit, and a photo-electric pulse conversion circuit that uses the differential amplification circuit capable of DC offset cancellation to convert an optical pulse signal to a corresponding electrical pulse.
Alternatively, it relates to a pulse shaping circuit that generates a shaped pulse signal whose logic changes in a manner similar to a rise and a fall of a base square-wave pulse signal, a pulse generation circuit that uses this pulse shaping circuit, and a photo-electric pulse conversion circuit that uses the pulse shaping circuit to convert an optical pulse signal to a corresponding electrical pulse.
2. Description of Related Art
(Related Art 1)
In a differential amplification circuit that amplifies an input signal and outputs a pair of differential output signals, a difference in reference voltages (hereinafter referred to also as a DC offset voltage) occurring between a pair of complementary differential output signals outputted from the differential amplification circuit, namely, between a non-inversion output signal and an inversion output signal presents at times a problem. Therefore, a differential amplification circuit provided with a DC offset cancellation circuit that cancels the DC offset voltage is proposed.
A photo-electric pulse conversion circuit 10 shown in FIG. 35 will be explained as an example. The photo-electric pulse conversion circuit 10 converts an optical pulse signal LT to an electrical pulse signal xRX. For example, it is used as a receiving circuit in IrDA communications and transmits an inversion electrical pulse signal xRX to a demodulator circuit at a later stage.
When it is used in such optical communications, the distance from a transmitter circuit (a light source) to a receiving circuit (photodiode PD) is not constant, and therefore there are various conditions. In some cases, the received optical pulse signal LT is very feeble due to a long distance, and in other cases, the received signal LT is extremely strong due to a short distance. As a result, a current input signal fluctuates from scores of nA to several mA which is several hundred times as large as the scores of nA. Even in such cases, it is necessary to reliably receive the optical pulse signal, to shape the waveform while accurately maintaining a pulse width thereof, and to send the resultant signal to the demodulator circuit at a later stage.
In the photo-electric pulse conversion circuit 10, when the photodiode PD receives the optical pulse signal LT that rises at a second timing t2 and falls at a first timing t1, a pulsating current signal I in flows according to the intensity of the light. An I-V conversion circuit IV converts this current signal Iin to a pair of complementary differential voltage signals, namely, a non-inversion voltage signal V1P that is in the same phase as the optical pulse signal LT and the current signal Iin and an inversion voltage signal V1M that is complementary thereto and outputs these signals. The waveforms of the differential voltage signals V1P, V1M when a large signal is inputted are slightly different from those when a small signal is inputted as shown in FIG. 36. When a small signal is inputted, the current signal Iin of the photodiode PD having a pulse width tpw that nearly corresponds to the optical pulse signal LT is obtained. When a large signal is inputted, however, the waveform has a dull rising edge and a dull falling edge though it has a generally square shape. This is because the electrical signal fails to accurately follow changes in the optical input. Particularly, since the falling edge after the first timing t1 falls slowly, the non-inversion voltage signal V1P also falls slowly as shown in FIG. 36.
The differential voltage signals V1P, V1M are then amplified by a first differential amplification circuit AMP1 provided with a DC offset cancellation circuit OFC indicated by dashed lines in FIG. 35 and a second differential amplification circuit AMP2. Then, as shown in FIG. 37, a reference voltage VREF according to an output VO of the amplifier is generated by a reference voltage generation circuit REFG and both signals are compared with each other by a comparison circuit CMP to obtain an inversion electrical pulse signal xRX which has the pulse width tpw corresponding to the optical pulse signal LT and which falls at the second timing t2 and rises at the first timing t1.
More specifically, an offset adding circuit (mixing circuit) OFP is used to mix the offset cancellation voltage VOC into the differential voltage signals V1P, V1M such that a negative feedback is performed, thereby generating second differential signals V2P, V2M which are amplified by the first differential amplification circuit AMP1 to output third differential signals V3P, V3M. In the DC offset cancellation circuit OFC, the third differential signals V3P, V3M are filtered by a low-pass filter LPF having characteristics of a cutoff frequency fc1 and a through rate SR1 to obtain the offset cancellation voltage VOC. Since the DC offset voltage occurring between the third differential signals V3P and V3M is negatively fed back in this manner, the DC offset voltage between the differential output terminals of the differential amplification circuit AMP1 can be canceled. If a DC offset voltage exists, an output VO from the second differential amplification circuit AMP2 fluctuates causing the pulse width obtained in the comparison circuit CMP to fluctuate. Thus, the pulse width of the inversion electrical pulse signal xRX obtained may become different from the optical pulse signal. By canceling the DC offset voltage, however, the inversion electrical pulse signal xRX having the pulse width tpw which accurately corresponds to the optical pulse signal can be obtained.
To obtain the inversion electrical pulse signal xRX having the accurate pulse width tpw, it is necessary to give the reference voltage VREF an appropriate time constant according to the magnitude of the output VO.
(Related Art 2)
It is possible to employ a differential amplification circuit provided with a DC offset cancellation circuit in the same manner also in a photo-electric pulse conversion circuit 20 with another configuration (see FIG. 39).
The photodiode PD receives the optical pulse signal LT that rises at the second timing t2 and falls at the first timing t1 to provide the current signal Iin also in this photo-electric pulse conversion circuit 20. However, the photo-electric pulse conversion circuit 20 uses, instead of the I-V conversion circuit IV, a differentiating I-V conversion circuit DIV to convert a waveform of the current signal Iin to a pair of complementary differential voltage signals VD1P, VD1M whose waveforms are similar to a differentiated waveform of the current signal Iin. The differential voltage signals VD1P, VD1M are then amplified by the differential amplification circuit AMP provided with the offset cancellation circuit OFC to output third differential signals VD3P, VD3M. The third differential signals VD3P and VD3M are compared with each other by the comparison circuit CMP and the inversion electrical pulse signal xRX is obtained.
In the photo-electric pulse conversion circuit 20, the differential voltage signals VD1P, VD1M whose waveforms are similar to a differentiated waveform of the current signal Iin are obtained, and are then amplified. A third differential signal VD3P and a third differential signal VD3M that sharply fall or rise at the first or the second timing t1, t2 are compared. It is therefore possible to accurately reproduce the pulse width tpw of the optical pulse signal LT in the obtained inversion electrical pulse signal xRX. In addition, the circuit has the advantage that there is no need of separately using the reference voltage generation circuit REFG to generate the reference voltage VREF according to the output VO as in related art 1 (see FIG. 35).
A DC offset voltage VOS of a small value may be added in the comparison circuit CMP to prevent a malfunction caused by noise.
(Related Art 3)
In the circuit shown in related art 2 (see FIG. 39), the pulse signal is once differentiated to obtain the differential signals, and using these differential signals, a pulse signal having the same pulse width as the original pulse signal is obtained. As a circuit of the same type, a photo-electric pulse conversion circuit 30 shown in FIG. 41 may be configured.
Namely, in the photo-electric pulse conversion circuit 30, the optical pulse signal LT that rises at the second timing t2 and falls at the first timing t1 is received by the photodiode PD and the current signal Iin is obtained. Then, the current signal Iin is converted to the corresponding voltage signal V1 and the resultant voltage signal is outputted by the I-V conversion circuit IV. The voltage signal V1 is then amplified by the differential amplification circuit AMP. Thereafter, a differentiating differential amplification circuit DAMP is used to differentiate and amplify the second differential signals V2P, V2M to output third differential signals VD3P, VD3M. In addition, an offset voltage VOS of a small value is added so that the reference voltage of the third non-inversion signal VD3P is relatively lower than the reference voltage of the third inversion signal VD3M. These third differential signals VD3P, VD3M are then compared with each other by the comparison circuit CMP to obtain the inversion electrical pulse signal xRX that falls at the second timing t2 and rises at the first timing t1. As mentioned above, the purpose of adding the offset voltage VOS is to prevent a malfunction caused by noise.
The current signal Iin and the voltage signal V1 have slightly dull waveforms that gradually fall after the first timing t1 (see FIG. 36) also in the photo-electric pulse conversion circuit 30. However, since the third differential signals VD3P, VD3M that sharply rise or fall at the first or the second timing t1, t2 are compared to obtain the inversion electrical pulse signal xRX, it is possible to accurately reproduce the pulse width tpw of the optical pulse signal LT in the inversion electrical pulse signal xRX. Furthermore, the circuit has the advantage that there is no need of separately using the reference voltage generation circuit REFG to generate the reference voltage VREF according to the output VO as in the circuit 10 shown in related art 1 (FIG. 35).
In the photo-electric pulse conversion circuit 10 of related art 1, however, the low-pass filter LPF with the cutoff frequency fc1 is used to provide a negative feedback control of the DC offset voltage. Therefore, not only DC components, but also low-frequency components of AC components contained in the pulse signal waveform are fed back. Namely, as shown in FIG. 38(a), if the DC offset voltage DCO exists between the third differential signals V3P and V3M of the first differential amplification circuit AMP1 and is negatively fed back, DC cancellation components are contained in the offset cancellation voltage VOC outputted from the low-pass filter LPF as shown in FIG. 38(b) and they function to cancel the DC offset voltage DCO. However, since the low-frequency AC components also pass through the low-pass filter LPF as mentioned above, the low-frequency AC components are also superposed on the offset cancellation voltage VOC as shown in FIG. 38(b).
The magnitude of these AC components contained in the offset cancellation voltage VOC gradually increases during a second period d2 from the second timing t2 to the first timing t1. During a first period d1 from the first timing t1 to the second timing t2, it gradually decreases to return to an original zero level which is maintained. This is because the AC components are contained in the second period d2 as can be easily understood from FIG. 38(a). The gradient of the graph showing the offset cancellation voltage VOC corresponds to the characteristics of the low-pass filter LPF (the cutoff frequency and the through rate), and the increasing gradient and the decreasing gradient become almost the same.
If the second period d2 is longer than the first period d1 as shown in FIG. 38(c), the AC components contained in the offset cancellation voltage VOC cannot decrease in the first period by the amount increased in the second period. As a result, the AC components contained in the offset cancellation voltage VOC gradually accumulate as shown in FIG. 38(d) (in this example, they gradually increase). Therefore, as a result of accumulated AC components being negatively fed back, the waveforms of the third differential signals V3P and V3M as the output from the first differential amplification circuit AMP1 are distorted. This could result in a malfunction or other problem when the inversion electrical pulse signal xRX is obtained in the comparison circuit CMP. Moreover, as AC components accumulate, the waveforms are distorted so as to gradually shift downward, and approach an upper limit value or a lower limit value of the third differential signals V3P, V3M. As a result, the dynamic range may become small and the signal amplitude may become small, and in extreme cases, the third differential signals V3P, V3M may disappear.
On the other hand, in the photo-electric pulse conversion circuit 20 according to related art 2, the current signal Iin as shown in FIG. 40(b) flows through the photodiode PD when receiving the optical pulse signal LT with the pulse width tpw as shown in FIG. 40(a). FIG. 40(b) shows a case in which a large optical pulse signal LT with a high intensity is inputted. In the first period d1, the current signal Iin forms a gradually decreasing long tail. The signal is then subjected to differentiation and I-V conversion performed by the differentiating I-V conversion circuit DIV to obtain the non-inversion voltage signal VD1P shown in FIG. 40(c) and the inversion voltage signal VD1M. These signals are then amplified by the differential amplification circuit AMP to obtain the third differential signals VD3P, VD3M [see FIG. 40(d)]. In this example, the amplitudes of the amplified third differential signals VD3P, VD3M are limited by the upper limit value or the lower limit value in the second period d2 and the first half of the first period d1. As a result, their waveforms are not similar to the waveform of the non-inversion voltage signal VD1P shown in FIG. 40(c). Unlike the photo-electric pulse conversion circuit 10 [see FIGS. 37(a) and(c)], this circuit employs a differentiated waveform and therefore the non-inversion voltage signal VD1P swings to the positive and negative directions (upward and downward) with respect to the reference voltage.
The low-pass filter LPF with the cutoff frequency fc1 (through rate SR1) is used also in the photo-electric pulse conversion circuit 20 according to related art 2. Therefore, not only the DC components but also the low-frequency components of the AC components are fed back. That is, the AC components are superposed on the offset cancellation voltage VOC.
The magnitude of the AC components contained in the offset cancellation voltage VOC gradually increases in the second period d2 from the second timing t2 to the first timing t1 as shown in FIG. 40(e) and gradually decreases in the first period d1 from the first timing t1 to the second timing t2. Unlike the photo-electric pulse conversion circuit 10 [see FIG. 38(b)], however, it continues decreasing in the first period d1. The gradient of the graph showing the offset cancellation voltage VOC corresponds to the characteristics of the low-pass filter LPF (the cutoff frequency and the through rate), and the increasing gradient and decreasing gradients become almost the same.
If the first period d1 is not equal to the second period d2 (if the duty ratio of the pulse is not 50%), the offset cancellation voltage VOC gradually fluctuates. If d1 greater than d2 as shown in FIG. 40, for example, the AC components contained in the offset cancellation voltage VOC gradually accumulate, causing the offset cancellation voltage VOC to gradually diminish as shown in FIG. 40(e).
As a result, the third differential signals VD3P, VD3M of the differential amplification circuit AMP are distorted and the third non-inversion signal VD3P approaches the upper limit value as shown in FIG. 40(f), for example. Therefore, a malfunction may occur when obtaining the inversion electrical pulse signal xRX in the comparison circuit CMP. Moreover, the waveforms approach the upper limit value or the lower limit value of the third differential signals VD3P, VD3M. As a result, the dynamic range may become small and the signal amplitude may become small and, in extreme cases, the third differential signals VD3P, VD3M may become disappear.
In the photo-electric pulse conversion circuit 30 according to related art 3, when the pulse width tpw of the base pulse signal, that is, the optical pulse signal LT becomes long, the gradually downward-going third non-inversion signal VD3P and the gradually upward-going third inversion signal VD3M can cross at a time tx after the second timing t2 as shown in FIG. 42. Then, the inversion electrical pulse signal xRX which is the output of the comparison circuit CMP is inverted. Therefore, as shown in the lower part of FIG. 42, there arises a problem that the pulse width of the inversion electrical pulse signal xRX, which should rise at the first timing t1, becomes shorter. Particularly, the pulse width tends to become shorter when the offset voltage VOS is made greater in an attempt to prevent a malfunction caused by noise.
If the comparison circuit CMP is given hysteresis characteristics (hysteresis voltage Vh) so as to satisfy Vh  greater than VOS as shown in FIG. 43, the third non-inversion signal VD3P and the third inversion signal VD3M do not cross between the second timing t2 and the first timing t1, and thus the correct pulse width tpw is obtained in the inversion electrical pulse signal xRX.
In the case where an arrangement is made to satisfy Vh  greater than VOS as described above and once the third inversion signal VD3M becomes smaller than the third non-inversion signal VD3P when the circuit is started or noise intrudes, the inversion electrical pulse xRX which is the output of the comparison circuit CMP is inverted, that is, the level of the inversion electrical pulse xRX becomes LOW level as shown in FIG. 44. This also causes the same effect as relatively decreasing the third non-inversion signal VD3P by the amount equal to hysteresis voltage Vh. Since the third inversion signal VD3M becomes smaller than the third non-inversion signal VD3P, the inversion electrical pulse signal xRX is fixed to LOW level even after the noise has disappeared and the inversion electrical pulse signal xRX does not fall at the second timing t2. It thereafter returns to HIGH level at the first timing t1. In this case, therefore, the optical pulse signal has not been properly received.
Thus, in the photo-electric pulse conversion circuits 20, 30 according to related arts 2, 3, it is difficult to prevent a malfunction due to noise or the like by setting the offset voltage VOS and the hysteresis voltage Vh to adequate values simultaneously.
In view of the problems arising from related arts 1, 2, it is an object of the invention to provide a DC offset cancellation circuit which is capable of canceling a DC offset voltage occurring between differential output signals of a differential amplification circuit, while preventing a signal waveform from being distorted due to accumulation of AC components, and a photo-electric pulse conversion circuit which is capable of generating an electrical pulse signal that accurately reproduces a rise and a fall timing of an optical pulse signal by canceling the DC offset voltage occurring between the differential output signals of the differential amplification circuit.
In addition, in view of the problems arising from related arts 2, 3, it is an object of the invention to provide a pulse shaping circuit which is capable of obtaining a non-inversion shaped pulse signal or an inversion shaped pulse signal that sharply rises or falls at a rise timing (a second timing) or a fall timing (a first timing) of a base pulse signal and accurately reproduces a pulse width of the base pulse signal, and in which less malfunctions are caused by noise or the like, and a photo-electric pulse conversion circuit which is capable of generating an electrical pulse signal that accurately reproduces a pulse width of an optical pulse signal.
The means for solving the problems according to a first aspect of the invention is a DC offset cancellation circuit which is inserted between a pair of differential output terminals and a pair of differential input terminals of a differential amplification circuit that amplifies a pair of differential input signals inputted to the differential input terminals and outputs a pair of differential output signals from the differential output terminals, and which cancels a DC offset voltage between the differential output signals. It is provided with a low-pass filter which performs low-pass filtration on the inputted differential output signals to output a filtered signal, a hold circuit which outputs a hold filtered signal instead of the filtered signal of the low-pass filter, and which outputs the hold filtered signal corresponding to the filtered signal of the low-pass filter at the time of changing the filtered signal to the hold filtered signal, a mixing circuit that outputs a pair of mixed differential input signals, which are generated by mixing the filtered signal or the hold filtered signal into the differential input signals such that a negative feedback is performed, to the differential input terminals of the differential amplification circuit, and a changeover circuit that alternately performs changeover to a filtering state in which the differential output signals are inputted to the low-pass filter and the filtered signal is outputted to the mixing circuit, and changeover to a hold state in which an input of the differential output signals to the low-pass filter are cut off and the hold filtered signal is outputted to the mixing circuit.
As explained in related arts 1, 2, when a configuration is made such that the DC offset voltage is negatively fed back using the low-pass, not only the DC offset voltage but also the AC components contained in the differential output signals, particularly the low-frequency components pass through the low-pass filter LPF and are negatively fed back to the differential input terminals. As a result, there may arise a problem that AC components gradually accumulate, the output signals are distorted, and the dynamic range of the output signals become small, and in extreme cases, the output signals disappear depending on the waveform and duty ratio of the input signal.
According to the invention, the hold circuit and the changeover circuit are provided in addition to the low-pass filter and the mixing circuit. By performing changeover between the low-pass filter and the hold circuit at a predetermined timing according to the signal waveform, it is possible to cancel DC offset, while preventing AC components from being accumulated. Specifically, for a period during which no or few AC components are contained in the signal, the low-pass filter is selected to perform DC offset cancellation. For a period during which many AC components are contained in the signal, on the other hand, the input to the low-pass filter is cut off to reduce effects of AC components on the low-pass filter, the negative feedback by the low-pass filter is stopped, and the hold circuit is selected to perform DC offset cancellation at the same level as that before the hold circuit is selected, while preventing AC components from being accumulated. More specifically, in terms of a signal inputted to the non-inversion input terminal of the differential amplifier, if a pulse signal having HIGH level and LOW level alternately or a signal including a train of signals with intermittent no-signal (LOW-level) periods there between is inputted, the low-pass filter is selected for the period during which a signal has LOW level or no signals exist, and the hold circuit is selected for the period during which a signal has HIGH level or a train of signals exists. Thus, AC components are effectively prevented from being accumulated.
It is preferable that the DC offset cancellation circuit according to the first aspect of the invention be a differential amplification circuit provided with a DC offset cancellation circuit inserted between the differential output terminal and the differential input terminal of the differential amplification circuit.
According to the differential amplification circuit provided with the DC offset cancellation circuit, since the DC offset cancellation circuit is inserted, the DC offset voltage is canceled and a differential output signal free from distortions due to accumulation of AC components can be obtained.
Furthermore, the photo-electric pulse conversion circuit that converts an optical pulse signal to a corresponding electrical pulse signal is preferably provided with a light-current conversion circuit that converts the optical pulse signal to a corresponding current signal and outputs the current signal, an I-V conversion circuit that converts the current signal to a pair of corresponding differential voltage signals and outputs the differential voltage signals, the differential amplification circuit provided with the DC offset cancellation circuit according to another form of the first aspect of the invention that amplifies the differential voltage signals and outputs the differential output signals, and a pulse generation circuit that outputs the electrical pulse signal based on the differential output signals.
If a DC offset voltage is occurring in the differential amplification circuit used in the photo-electric pulse conversion circuit, the rise and fall timings of the electrical pulse signal generated by the pulse generation circuit do not match the rise and fall timings of the corresponding optical pulse signal, which may result in faulty communications or other problems.
On the other hand, the photo-electric pulse conversion circuit according to the invention employs the differential amplification circuit provided with the DC offset cancellation circuit. More specifically, it uses the differential amplification circuit with the DC offset cancellation circuit inserted between the differential output terminals and the differential input terminals. This ensures that the DC offset voltage is properly canceled to perform differential amplification, eliminating the possibility that the rise and fall timings of the electrical pulse signal may not match those of the optical pulse signal due to accumulation of AC components.
Another means for solving the problems according to a second aspect of the invention is a photo-electric pulse conversion circuit that converts an optical pulse signal of a generally square wave shape to at least either a non-inversion electrical pulse signal that falls at a first timing at which the optical pulse signal falls and rises at a second timing at which the optical pulse signal rises or an inversion electrical pulse signal that rises at the first timing and falls at the second timing. It is provided with a light-current conversion circuit that converts the optical pulse signal to a corresponding current signal and outputs the current signal, an I-V conversion circuit that converts the current signal to a pair of corresponding differential voltage signals and output the differential voltage signals, a differential amplification circuit that amplifies the differential voltage signals and outputs a pair of differential output signals, and a pulse generation circuit that outputs at least either the non-inversion electrical pulse signal or the inversion electrical pulse signal based on the differential output signals. The differential amplification circuit is provided with a low-pass filter which filters the differential output signals to output a filtered signal, a hold circuit which outputs a hold filtered signal instead of the filtered signal of the low-pass filter, and which outputs the hold filtered signal corresponding to the filtered signal of the low-pass filter at the time of changing the filtered signal to the hold filtered signal, a mixing circuit which outputs a pair of mixed differential input signals, which are generated by mixing the filtered signal or the hold filtered signal into the differential voltage signals such that a negative feedback is performed, to the differential input terminals of the differential amplification circuit, and a changeover circuit that performs changeover to a filtering state in which the differential output signals is inputted to the low-pass filter and the filtered signal is outputted to the mixing circuit at the first timing, and performs changeover to a hold state in which an input of the differential output signals to the low-pass filter are cut off and the hold filtered signal is outputted to the mixing circuit at the second timing according to the inputted non-inversion electrical pulse signal or the inversion electrical pulse signal.
This photo-electric pulse conversion circuit is provided with the light-current conversion circuit, the I-V conversion circuit, the differential amplification circuit, and the pulse generation circuit. The differential amplification circuit is provided with the low-pass filter that outputs the filtered signal, the hold circuit that outputs the hold filtered signal, the mixing circuit that outputs the mixed differential input signals, which is generated by mixing the filtered signal or the hold filtered signal into the differential input signals, to the differential amplification circuit, and the changeover circuit that performs changeover between the filtering state and the hold state according to the non-inversion electrical pulse signal or the inversion electrical pulse signal.
In the differential amplification circuit, therefore, the DC offset voltage is canceled and AC components do not accumulate. Therefore, a non-inversion electrical pulse signal or an inversion electrical pulse signal that accurately reproduces the rise and fall timings of the optical pulse signal can be generated.
As the I-V conversion circuit, any circuit may be employed as long as it performs current-to-voltage conversion of a current signal and outputs a pair of corresponding differential voltage signals. Possible circuits therefore include a circuit that performs amplification and current-to-voltage conversion concurrently and a circuit that performs amplification after current-to-voltage conversion.
Still another means for solving the problems according to a third aspect of the invention is a DC offset cancellation circuit which is inserted between a pair of differential output terminals and a pair of differential input terminals of a differential amplification circuit that amplifies a pair of differential input signals inputted to the differential input terminals thereof and output a pair of differential output signals from the differential output terminals thereof, and which cancels a DC offset voltage between the differential output signals. It is provided with a low-pass filter that performs low-pass filtration on the differential output signals to output a filtered signal, a mixing circuit that outputs a pair of mixed differential input signals, which are generated by mixing the filtered signal into the differential input signals such that a negative feedback is performed, to the differential input terminals of the differential amplification circuit, and a characteristics changing circuit that changes a cutoff frequency and a through rate of the low-pass filter.
Since this DC offset cancellation circuit is provided with the low-pass filter and the mixing circuit, low-frequency AC components, in addition to DC components, are negatively fed back through the low-pass filter. Because of the characteristics changing circuit provided therein, however, the DC offset cancellation circuit can change the cutoff frequency and the through rate of the low-pass filter by means of the characteristics changing circuit. By changing the cutoff frequency and the through rate according to the waveform and the like of the input signal, therefore, it is possible to adjust a rate of increase or decrease of AC components, and to eliminate or control accumulation of AC components, thereby preventing a problem that the differential output signals are distorted due to accumulation of AC components or other problems.
The characteristics changing circuit may change the cutoff frequency and the through rate from one value to another value in a step-by-step manner, or may change them continuously.
It is preferable to configure a differential amplification circuit provided with the DC offset cancellation circuit according to the third aspect of the invention inserted between the differential output terminals and the differential input terminals of the differential amplification circuit.
According to the differential amplification circuit provided with the DC offset cancellation circuit, since the DC offset cancellation circuit is inserted, the DC offset voltage can be canceled and a pair of differential output signals in which a distortion due to accumulation of AC components is prevented can be obtained.
Furthermore, the photo-electric pulse conversion circuit that converts an optical pulse signal to a corresponding electrical pulse signal is preferably provided with a light-current conversion circuit that converts the optical pulse signal to a corresponding current signal and outputs the current signal, a high-pass I-V conversion circuit that converts the current signal to a pair of corresponding differential voltage signals and outputs the differential voltage signals through a capacitive coupling capacitor or a differentiating I-V conversion circuit that converts the current signal to a pair of differential voltage signals with a waveform similar to that of signals which are obtained by differentiation of the current signal, a differential amplification circuit provided with the DC off set cancellation circuit according to another form of the third aspect of the invention that amplifies the differential voltage signals as the differential input signals and output the differential output signals, and a pulse generation circuit that outputs the electrical pulse signal based on the differential output signals.
If a DC offset voltage is occurring in the differential amplification circuit used in the photo-electric pulse conversion circuit, since the rise and fall timings of the electrical pulse signal generated by the pulse generation circuit do not match the rise and fall timings the corresponding optical pulse signal, it is impossible to obtain an electrical pulse signal having a pulse width corresponding to the pulse width of the optical pulse signal, which may result in faulty communications or other problems.
On the other hand, the photo-electric pulse conversion circuit according to the invention employs the differential amplification circuit provided with the DC off set cancellation circuit. More specifically, it uses the differential amplification circuit with the DC offset cancellation circuit inserted between the differential output terminals and the differential input terminals. This allows the DC offset voltage to be reliably canceled to perform differential amplification and prevents AC components from accumulating, thereby inhibiting mismatch between the rise and fall timings of the electrical pulse signal and those of the optical pulse signals due to accumulation of AC components.
A further means for solving the problems according to a fourth aspect of the invention is a photo-electric pulse conversion circuit that converts an optical pulse signal of a generally square wave shape to at least either a non-inversion electrical pulse signal that falls at a first timing at which the optical pulse signal falls and rises at a second timing at which the optical pulse signal rises or an inversion electrical pulse signal that rises at the first timing and falls at the second timing. It is provided with a light-current conversion circuit that converts the optical pulse signal to a corresponding current signal and outputs the current signal, either a high-pass I-V conversion circuit that converts the current signal to a pair of corresponding differential voltage signals and outputs the differential voltage signals through a capacitive coupling capacitor or outputs the differential voltage signals after passing it through the capacitor and then amplifying it, or a differentiating I-V conversion circuit that converts the current signal to a pair of differential voltage signals with a waveform similar to that of a signal which is obtained by the differentiation of the current signal and outputs the differential voltage signals, a differential amplification circuit that amplifies the differential voltage signals and outputs the differential output signals, and a pulse generation circuit that outputs at least either the non-inversion electrical pulse signal or the inversion electrical pulse signal based on the differential output signals. Furthermore, the differential amplification circuit is provided with a low-pass filter that performs low-pass filtration on the inputted differential output signals to output a filtered signal, a mixing circuit that outputs a pair of mixed differential input signals, which is generated by mixing the filtered signal into the differential voltage signals such that a negative feedback is performed, to a pair of differential input terminals of the differential amplification circuit, and a characteristics changeover circuit that performs changeover to a first state in which the cutoff frequency is a first cutoff frequency fc1 and the through rate is a first through rate SR1 at the first timing, and performs changeover to a second state in which the cutoff frequency is a second cutoff frequency fc2 and the through rate is a second through rate SR2 at the second timing, according to the inputted non-inversion electrical pulse signal or the inversion electrical pulse signal.
This photo-electric pulse conversion circuit is provided with the light-current conversion circuit, the high-pass I-V conversion circuit or the differentiating I-V conversion circuit, the differential amplification circuit, and the pulse generation circuit. Moreover, the differential amplification circuit is provided with the low-pass filter that outputs the filtered signal, the mixing circuit that outputs the mixed differential input signals to the differential amplification circuit, and the characteristics changeover circuit that changes the characteristics of the low-pass filter according to the non-inversion electrical pulse signal or the inversion electrical pulse signal.
In the differential amplification circuit, therefore, the DC offset voltage is canceled and a non-inversion electrical pulse signal or an inversion electrical pulse signal that accurately reproduces the rise and fall timings of the optical pulse signal can be generated.
Examples of the high-pass I-V conversion circuit include a circuit that performs current-to-voltage conversion on a current signal to produce a pair of differential voltage signals and outputs the signals through a capacitive coupling capacitor and a circuit that outputs the signal through the capacitor and further amplification. Also included is a circuit configured so as to perform amplification and conversion concurrently, or to perform amplification following conversion when converting the current signal to the differential voltage signals for outputting the amplified differential voltage signals through the capacitive coupling capacitor.
As the differentiating I-V conversion circuit, any circuit may be employed as long as it is capable of converting the current signal to a pair of differential voltage signals with a waveform similar to that of signals which are obtained by differentiation of the current signal and outputting the differential voltage signals. A circuit that performs amplification and conversion concurrently, or performs amplification following conversion is also included.
A still further means for solving the problems according to a fifth aspect of the invention is a pulse shaping circuit that performs logic processing on a pair of complementary pulse differentiated differential input signals obtained by subjecting a base pulse signal of a generally square wave shape to differentiation or high-pass filtration and obtains at least either a non-inversion shaped pulse signal that falls at a first timing at which the base pulse signal falls and rises at a second timing at which the base pulse signal rises or an inversion shaped pulse signal that rises at the first timing and falls at the second timing. The pulse shaping circuit is provided with an offset-added signal generation circuit that uses the pair of pulse complimentary differentiated differential input signals, that is, a non-inversion first signal and an inversion second signal, to generate a non-inversion fifth signal corresponding to the non-inversion first signal and an inversion fourth signal corresponding to the inversion second signal by adding an offset voltage so that a fourth reference voltage of the inversion fourth signal is relatively higher than a fifth reference voltage of the non-inversion fifth signal by an amount equivalent to a first offset voltage and to generate a non-inversion third signal corresponding to the non-inversion first signal and an inversion sixth signal corresponding to the inversion second signal by adding an offset voltage so that a sixth reference voltage of the inversion sixth signal is relatively lower than a third reference voltage of the non-inversion third signal by an amount equivalent to a second offset voltage. The pulse shaping circuit is also provided with a first comparison circuit that compares the non-inversion fifth signal with the inversion fourth signal to obtain a seventh signal that rises or an inversion seventh signal that falls at the second timing, a second comparison circuit that compares the non-inversion third signal with the inversion sixth signal to obtain an eighth signal that rises or an inversion eighth signal that falls at the first timing, and a logic processing circuit that obtains at least either the non-inversion shaped pulse signal or the inversion shaped pulse signal based on the seventh signal and the eighth signal or the inversion seventh signal and the inversion eighth signal.
The pulse shaping circuit according to the invention is provided with the offset-added signal generation circuit that uses the pair of pulse differentiated differential input signals, that is, the non-inversion first signal and the inversion second signal, to generate the non-inversion third signal, the non-inversion fifth signal, the inversion fourth signal, and the inversion sixth signal by adding the offset voltage equivalent to the first and the second offset voltages (xcex94Vof1, xcex94Vof2). The pulse shaping circuit according to the invention is also provided with the first comparison circuit that obtains the seventh signal or the inversion seventh signal, the second comparison circuit that obtains the eighth signal or the inversion eighth signal, and the logic processing circuit that obtains at least either the non-inversion shaped pulse signal or the inversion shaped pulse signal. Since the seventh signal that rises at the rise timing of the base pulse signal and the eighth signal that rises at the fall timing of the base pulse signal are separately obtained as described above, it is possible to obtain the non-inversion shaped pulse signal or the inversion shaped pulse signal that sharply rises or falls at the rise timing (the second timing) or the fall timing (the first timing) of the base pulse signal and accurately reproduces the pulse width of the base pulse signal. Furthermore, since the signal processing can be performed by adding the adequate first and second offset voltages xcex94Vof1, xcex94Vof2 regardless of the hysteresis voltages of the first and the second comparison circuits, malfunctions due to noise or the like can be reduced.
In addition, it is preferable that a pulse generation circuit for obtaining at least either the non-inversion shaped pulse signal or the inversion pulse signal from the base pulse signal of a generally square wave shape be provided with a differentiated differential signal generation circuit that performs differentiation or high-pass filtration on the base pulse signal of the generally square wave shape to generate the pair of complementary pulse differentiated differential input signals and the pulse shaping circuit according to the fifth aspect of the invention that uses the pulse differentiated differential input signals inputted thereto to obtain at least either the non-inversion shaped pulse signal or the inversion shaped pulse signal.
Since this pulse generation circuit is provided with the differentiated differential signal generation circuit and the pulse shaping circuit, a non-inversion shaped pulse signal or an inversion shaped pulse signal that accurately reproduces a base pulse signal of a generally square wave shape can be obtained. Moreover, malfunctions due to noise or the like can also be reduced.
In addition, it is preferable that a photo-electric pulse conversion circuit that converts an optical pulse signal to a corresponding electrical pulse signal be provided with a light-current conversion circuit that converts the optical pulse signal to a corresponding current signal and outputs the current signal, an I-V conversion circuit that converts the current signal to a corresponding pulse voltage signal of a generally square wave shape, and the pulse generation circuit according to another form of the fifth aspect of the invention that uses the pulse voltage signal as the base pulse signal to obtain at least either the non-inversion shaped pulse signal or the inversion shaped pulse signal.
This photo-electric pulse conversion circuit is provided with the pulse generation circuit, in addition to the light-current conversion circuit and the I-V conversion circuit. It is therefore possible to obtain a non-inversion shaped pulse signal or an inversion shaped pulse signal that accurately reproduces the pulse width of an optical pulse signal.
A yet further means for solving the problems according to a sixth aspect of the invention is a photo-electric pulse conversion circuit that converts an optical pulse signal of a generally square wave shape to at least either a non-inversion electrical pulse signal that falls at a first timing at which the optical pulse signal falls and rises at a second timing at which the optical pulse signal rises or an inversion electrical pulse signal that rises at the first timing and falls at the second timing. It is provided with a light-current conversion circuit that converts the optical pulse signal to a corresponding current signal and outputs the current signal, an I-V conversion circuit that converts the current signal to a corresponding pulse voltage signal of a generally square wave shape and outputs the pulse voltage signal, a differentiated differential signal generation circuit that performs differentiation or high-pass filtration on the pulse voltage signal to generate a pair of complementary pulse differentiated differential input signals, and a pulse shaping circuit that uses the pulse differentiated differential input signals inputted thereto to obtain at least either the non-inversion shaped electrical pulse signal or the inversion shaped electrical pulse signal. The pulse shaping circuit is provided with an offset-added signal generation circuit that uses the pair of pulse differentiated differential input signals, that is, a non-inversion first signal and an inversion second signal, to generate a non-inversion fifth signal corresponding to the non-inversion first signal and an inversion fourth signal corresponding to the inversion second signal by adding an offset voltage so that a fourth reference voltage of the inversion fourth signal is relatively higher than a fifth reference voltage of the non-inversion fifth signal by an amount equivalent to a first offset voltage, and to generate a non-inversion third signal corresponding to the non-inversion first signal and an inversion sixth signal corresponding to the inversion second signal by adding an offset voltage so that a sixth reference voltage of the inversion sixth signal is relatively lower than a third reference voltage of the non-inversion third signal by an amount equivalent to a second offset voltage. The pulse shaping circuit is also provided with a first comparison circuit that compares the non-inversion fifth signal with the inversion fourth signal to obtain a seventh signal that rises or an inversion seventh signal that falls at the second timing, a second comparison circuit that compares the non-inversion third signal with the inversion sixth signal to obtain an eighth signal that rises or an inversion eighth signal that falls at the first timing, and a logic processing circuit that obtains at least either the non-inversion shaped electrical pulse signal or the inversion shaped electrical pulse signal based on the seventh signal and the eighth signal or the inversion seventh signal and the inversion eighth signal.
The photo-electric pulse conversion circuit according to the invention is provided with the light-current conversion circuit, the I-V conversion circuit, the differentiated differential signal generation circuit, and the pulse shaping circuit. The pulse shaping circuit is provided with the offset-added signal generation circuit that uses the pair of pulse differentiated differential input signals, that is, the non-inversion first signal and the inversion second signal, to generate the non-inversion third signal, the non-inversion fifth signal, the inversion fourth signal, and the inversion sixth signal by adding the offset voltage equivalent to the first and the second offset voltages xcex94Vof1, xcex94Vof2. The pulse shaping circuit is also provided with the first comparison circuit that obtains the seventh signal or the inversion seventh signal, the second comparison circuit that obtains the eighth signal or the inversion eighth signal, and the logic processing circuit that obtains at least either the non-inversion shaped pulse signal or the inversion shaped pulse signal.
Since the seventh signal that rises at the rise timing of the optical pulse signal and the eighth signal that rises at the fall timing of the optical pulse signal are separately obtained as described above, it is possible to obtain the non-inversion shaped electrical pulse signal or the inversion shaped electrical pulse signal that sharply rises or falls at the rise timing (the second timing) or the fall timing (the first timing) of the optical pulse signal and, in addition, accurately reproduces the pulse width of the optical pulse signal. Furthermore, since the circuit permits signal processing to be performed by adding the adequate first and second offset voltages xcex94Vof1, xcex94Vof2 regardless of the hysteresis voltages of the first and the second comparison circuits, malfunctions due to noise or the like can be reduced.
The above and further objects and novel features of the invention will more fully appear from following detailed description when the same is read in connection with the accompanying drawings. It is to be expressly understood, however, that the drawings are for the purpose of illustration only and not intended as a definition of the limits of invention.